Semiconductor laser diodes find application in many different lightwave communication systems. A reduction in the temperature sensitivity of the laser characteristics is an important consideration for such systems. Fiber-to-the-home and fiber-in-the-loop optical networks for example, require the lasers to be low cost and to be uncooled; i.e., to operate without thermoelectric (TE) cooling. Uncooled lasers have several advantages: reduced size and cost of the packaging, and elimination of the reliability issues associated with the TE cooler itself.
Of particular interest is the uncooled MQW laser which typically includes an active region formed by a multiplicity of relatively smaller bandgap quantum well layers interleaved with larger bandgap barrier layers. Under forward bias electrons and holes are injected into the quantum wells where they undergo radiative recombination at a center wavelength characteristic of the effective bandgap energy, which is determined by the quantum well material, the well thickness and other factors. The barriers serve, in part, to confine the injected carriers to the wells. Frequently, MQW lasers are incorporated into a separate confinement heterostructure (SCH) which serves, in part, to provide optical confinement of the lasing mode. In an SCH laser the active region is bounded by a pair of inner cladding layers (sometimes referred to as waveguide layers), and the latter are bounded by a pair of outer cladding layers.
The threshold current density of all laser diodes increases exponentially with increasing temperature. The empirical relationship between threshold current density and temperature is given by equation (1): EQU J=J.sub.0 exp {T/T.sub.o } (1)
where J.sub.0 and T.sub.o are mathematical constructs, and T.sub.o is often referred to as the characteristic temperature of the laser. Because of the exponential relationship, a small change in T.sub.o produces a much larger shift in the threshold current density. In general, however, increasing T.sub.o is desirable. Several parameters are known to affect T.sub.o : waveguide material composition, strain energy, and dopant concentration. See, respectively, S. Seki, et al, IEEE Photon. Technol. Lett., Vol. 7, No. 8, pp. 839-841 (1995), P. J. A. Thijs, et al, IEEE J. Quantum Electron., Vol. 30, No. 2, pp. 477-499 (1994), and G. L. Belenky, et al, IEEE Photon. Technol. Lett., Vol. 9, No. 12, pp. 1558-1560 (1997), all of which are incorporated herein by reference.
Recently Belenky et al., supra, reported that the addition of Zn to the entire MQW active region of a strained, InGaAsP/InP MQW laser produced a small improvement in the temperature dependence of the laser threshold. Although Zn shows strong p-type conductivity in InGaAsP, it has a much lower solubility in InP than it does in InGaAsP. This fact is important because MQW lasers operating at center wavelengths of 1.3 .mu.m and 1.55 .mu.m lasers utilize InP as outer cladding layer material, and InGaAsP as inner cladding (waveguide), barrier and quantum well material. Zn placed in the InP outer cladding layers tends to be wicked-up by the active region of the device. Furthermore, because the quantum well layers are required to have a higher Ga and As content than that of the waveguide or barrier layers (so that the bandgap of the quantum well layers is less than that of the waveguide or barrier layers), Zn will preferentially segregate within them. (In general, Zn solubility increases with increasing Ga and As content.) Consequently, anomalous interstitial diffusion is often observed in the MQW active region and in the p-type cladding layer of Zn-doped MQW lasers. The presence of Zn (or any p-type dopant) in the quantum wells can lead to a substantial increase in the threshold current density and reduction in differential quantum efficiency (through enhanced internal losses). Precise control over the hole distribution is necessary in order to avoid such undesirable device characteristics (e.g., internal losses, threshold current density, quantum efficiency and reduced lifetime). The p-dopant distribution is among the most important for control of these parameters since the absorption losses are primarily dependent upon the hole concentration. (See, for example, C. H. Henry et al., IEEE J. Quantum Electr., Vol. 19, No. 6, pp. 947-952 (1983), which is also incorporated herein by reference).